Quadrature amplitude modulation QAM is a digital modulation method which is particularly suitable for transmitting high data rates and combined ASK (Amplitude Shift Keying) and PSK (Phase Shift Keying), i.e. the signal carrier is modulated in amplitude and phase. The amplitude and the frequency or phase of a harmonic oscillation are modulated by two different time functions. In QAM, different variants are possible. In a 64-QAM, 6 bits per data symbol are coded. With increasing level (16-, 64-, 256-, 1 024-QAM) the bandwidth efficiency increases but so does the signal/noise power ratio required for reliable transmission.
FIG. 1 shows a first QAM receiver according to the prior art. The data coming from a data source are transmitted by a transmitter via a transmission channel to the QAM receiver and delivered by this QAM receiver to a downstream data sink. During the data transmission via the real data transmission channel, the received signal, as a rule, exhibits linear distortion and an additional noise component. This noise component can be modeled by additive white Gaussian noise (AWGN). The QAM receiver has the task of reconstructing the bit sequence of the data source from the received signal. For this purpose, the received analog signal is first converted into a digital signal by an analog/digital converter ADC and then delivered to a mixing stage. A downstream receiving filter suppresses possible interference signals outside the transmission frequency band. The reliability of detection is increased by suitably dimensioning a matched filter (MF). The matched filter is specially adapted to a basic transmit pulse so that the greatest possible signal/noise power ratio (SNR) is achieved at the detection times. The impulse response of the matched filter MF is usually equal to the basic transmit pulse mirrored in time or displaced by one bit period. The matched filter is a digital receiving filter inside the receiver which is adapted to a transmit filter inside a transmitter, in such a manner that the amplitude of the received signal is maximum at the sampling times. The matched filter MF can be adaptively designed so that it can be adapted to the transmission channel. An adaptive equalizer which compensates for the distortion of the transmission channel can be provided before or after the matched filter.
The output signal of the matched filter MF is supplied to a carrier phase detector which is provided for detecting the carrier phase of a received digital signal. The carrier phase detector TPD delivers a carrier phase detection deviation signal TP to a downstream digital loop filter. The digital loop filter and the downstream NCO (numerically controlled oscillator) supply a digital control value for the mixing stage.
The QAM receiver according to the prior art, shown in FIG. 1, is constructed in one stage. Frequency and phase estimation are done in one stage. This has the disadvantage that a predetermined frequency range must be scanned for the carrier frequency of the received signal in a great number of small search steps in order to ensure that the target value is located within a narrow frequency capture range. The conventional QAM receiver shown in FIG. 1, therefore, needs a relatively large amount of time for such a search process.
For this reason, the QAM receiver according to the prior art shown in FIG. 2 was proposed. Such a QAM receiver is described in German patent application 101 33 898.8. The QAM receiver according to the prior art, shown in FIG. 2, is constructed in two stages. The QAM receiver contains a carrier frequency loop for detecting a carrier frequency of the received signal in a first carrier frequency capture range followed by a carrier phase loop for detecting a carrier phase of the received signal in a second carrier frequency capture range. Compared with the single-stage QAM receiver shown in FIG. 1, separating the frequency and phase estimation has the advantage that the frequency estimation by the carrier frequency loop has a large carrier frequency capture range.
The carrier frequency loop of the conventional QAM receiver shown in FIG. 2 has a carrier frequency detector which follows the two matched filters MF. The carrier frequency detector generates a carrier frequency deviation detection signal TF which is supplied to a numerically controlled oscillator NCO via a digital loop filter. The NCO delivers a control signal to the mixing stage.
The downstream carrier phase loop is constructed in a similar manner and contains a carrier phase detector TPD which generates a carrier phase deviation detection signal TP. The carrier phase deviation detection signal TP is also supplied to a loop filter which digitally filters the carrier phase deviation detection signal TP and delivers it to a downstream controlled oscillator NCO. The NCO forms a control signal which is supplied to a further mixing stage.
FIG. 3 is a diagram for explaining the operation of the conventional QAM receiver shown in FIG. 2. If the carrier frequency of the received signal is at a nominal frequency fENOM, the carrier frequency loop generates a carrier frequency deviation detection signal TFIDEAL having the value zero. However, due to interference, the carrier frequency loop generates a real carrier frequency deviation detection signal TFREAL which has an offset as shown in FIG. 3c. The frequency error after the frequency estimation can have various causes, namely intrinsic noise of the frequency detector, amplitude phase errors in the transmission channel or, for example, a wrongly dimensioned anti-aliasing filter. In all these cases, the characteristic of the frequency detector, which forms an S curve, is distorted in such a manner that the detector output signal does not provide a frequency offset even though there is a frequency offset present. A resultant controlled variable thus has the value zero for a frequency value which deviates from the ideal frequency estimate (FENOM). The real carrier frequency deviation detection signal TFREAL thus has a frequency offset.
If the deviation or the error of the frequency estimation by the carrier phase loop is too great, the subsequent phase estimation of the carrier phase of the received signal by the carrier phase loop can no longer compensate for the remaining frequency error. If, as shown in the example of FIG. 3, the zero value of the real carrier frequency deviation detection signal is located outside the carrier frequency capture range, shown in FIG. 3d, of the carrier phase loop, the carrier phase loop can no longer compensate for this considerable frequency error and the conventional QAM receiver shown in FIG. 2 cannot receive a received signal. The carrier frequency capture range of the carrier phase loop depends on the signal/noise ratio SNR of the complete transmission system and can only be extended within narrow limits.